It is well known today that there exists a world wide energy crisis as a result of which scientists are constantly seeking new forms of energy, as well as more efficient ways of utilizing known forms of energy. The heart of the energy crisis is, as generally accepted, that presently available sources of energy are dwindling at a rate faster than that at which alternate forms of energy are being made available, in commercially usable form. Solar energy, the heat which is present in the sun's rays, is a virtually inexhaustible supply of energy and, if it could be economically concentrated in large quantities, would completely solve the problem of the dwindling sources of energy which are more readily available. Although there are substantial quantities of fossil fuels such as natural gas, oil and coal, these sources of energy are virtually irreplaceable and will eventually be exhausted. Wind and water energy are abundant and virtually inexhaustible, but are difficult to harness in large quantities. Geothermal energy is still being developed and is not commercially practible as yet, and nuclear energy is obviously fraught with problems too numerous to mention here.
Accordingly, the sun represents a source of energy which is not only vast and inexhaustible, but also, from a purely technical standpoint, one which is very simple to convert from mere "sunshine" into a more usable form of high grade energy. In fact, solar energy is one of the few forms of heat energy which can be utilized to great advantage in its natural form, that is, without converting it to another form. Radiation from the sun is manifested on earth in a variety of forms of energy, the two principal forms being heat and visible light, although there are many types of radiation other than those within the visible spectrum. The heat which is generated on earth from solar radiation is a usable form of energy that can be concentrated and utilized very simply and directly in the form of heat. For example, it has long been known that, if the sun's rays are focused through a magnifying glass or a suitable lens onto a small area or spot, the heat energy generated on that spot is sufficient to cause combustion of certain materials, such as wood or paper. Thus it is well known that, by concentrating the rays of the sun, the high temperature heat thereby generated from the solar radiation can be utilized effectively in that form.
A major obstacle to the efficient commercial utilization of solar energy on a practical basis is that a tremendously large area of solar radiation must be concentrated into a relatively small area in order to produce the high temperatures required to generate practical quantities of heat. The problem, therefore, is to provide a commercially economic way of concentrating the sun's rays over large areas and of redirecting those rays to a location where they are concentrated on a device which can collect and utilize the tremendous amounts of heat which will be generated thereby. Stated differently, the problem is simply one of concentrating a large quantity of "sunshine" so as to generate at one location a sufficiently large quantity of usable high grade heat that the entire effort in terms of cost of hardware and maintenance becomes commercially feasible. This problem has been solved to a large extent by a variety of apparatuses which have the capability of reflecting large quantities of solar radiation and concentrating the radiation on a receiver. One such apparatus, called a heliostat, includes a large reflecting panel, or a plurality of smaller ones, and is capable of appropriate movement so that it can track the relative movement of the sun during the day and generally continuously reflect the rays of the sun to a distant stationary receiver so as to concentrate those rays on the receiver so that a large quantity of heat is generated in a relatively small area. Another such apparatus is called a dish concentrator and is generally structurally and functionally similar to a heliostat except that the dish concentrator typically has a plurality of small reflecting panels which are capable of focusing the rays of the sun on a receiver mounted at relatively close range on the dish concentrator itself. To avoid unnecessary duplication, the panels will be further described in connection with a typical heliostat as illustrative of the environment in which the reflecting panels are used.
Known heliostats generally comprise a large, heavy base anchored into the ground, a large frame mounted on the base which is movable both in azimuth and elevation, and a plurality of large reflecting glass mirror panels mounted on the frame and movable therewith. A typical heliostat might have a reflecting surface measuring 22 feet wide by 24 feet high, and would be made up of 12 individual panels, each 4 feet wide by 11 feet long and constructed of 1/4" thick glass with a silver reflective coating on the rear surface of each glass panel. Each panel must be very solidly braced by sufficiently strong structural steel members, in order to provide adequate support for the glass panel, which presents two major problems. One is that glass has a high degree of fragility or a low modulus and must be adequately supported to accommodate constantly varying wind forces. The other is that glass is very brittle and will accept only very small lateral, bending or twisting forces. These factors necessitate the use of very strong bracing, in the absence of which the glass panels would break. In addition, all of the panels must be mounted on the frame so that collectively they form a very slightly concave surface, which will focus all of the sun's rays impinging on the 528 square feet of reflecting area onto a distant receiver, that is relatively quite small in area, and several hundred feet above the ground. It is apparent that such a device, having a glass surface area 22 feet wide and as high as a three story house, must be able to withstand enormous forces from wind, hail, hurricanes, and perhaps even earthquakes. It will also be apparent that the bracing necessary to rigidly maintain the flat, planar configuration of each glass mirror panel and the frame necessary to support all of the panels against their own weight and against external forces, is of a tremendous magnitude, the entire assembly weighing many tons, and therefore very costly to manufacture and assemble. The magnitude of this may be further appreciated by understanding that a single solar energy installation may involve many thousands of such structures located within a 200-500 acre area, all aimed at the same receiver, with consequent total costs for such an installation reaching into the hundreds of millions of dollars.
It can thus be appreciated that a major advantage will have been provided if the cost of each heliostat can be significantly reduced. The utility of heliostats in terms of practical commercial acceptance is measured primarily by the cost per square meter of the reflecting surface, the degree of solar reflectance, its maintainable pointing accuracy and the life expectancy of the unit. Therefore, depending upon the type of construction involved, a particular design of heliostat may or may not be commercially acceptable based on these factors. The lower the cost per square meter of high quality reflective surface, the more a particular design of heliostat can be utilized on a commercial scale.